def shoot(self):
	self.bullet.being = True
	self.bullet.turn(self.direction_x, self.direction_y)
	if self.direction_x == 0 and self.direction_y == -1:
		self.bullet.rect.left = self.rect.left + 20
		self.bullet.rect.bottom = self.rect.top - 1
	elif self.direction_x == 0 and self.direction_y == 1:
		self.bullet.rect.left = self.rect.left + 20
		self.bullet.rect.top = self.rect.bottom + 1
	elif self.direction_x == -1 and self.direction_y == 0:
		self.bullet.rect.right = self.rect.left - 1
		self.bullet.rect.top = self.rect.top + 20
	elif self.direction_x == 1 and self.direction_y == 0:
		self.bullet.rect.left = self.rect.right + 1
		self.bullet.rect.top = self.rect.top + 20
	else:
		raise ValueError('myTank class -> direction value error.')
	if self.level == 0:
		self.bullet.speed = 8
		self.bullet.stronger = False
	elif self.level == 1:
		self.bullet.speed = 12
		self.bullet.stronger = False
	elif self.level == 2:
		self.bullet.speed = 12
		self.bullet.stronger = True
	elif self.level == 3:
		self.bullet.speed = 16
		self.bullet.stronger = True
	else:
		raise ValueError('myTank class -> level value error.')